Well I say no equipment but we do have a rowing machine.
Unfortunately I find that with what I'm sure seems to be good technique it still gives me a bad back. I've had no back injury but I do have to look after my back 24/7 by either stretching my hamstrings daily to allow sitting withuot putting pressure on the lower back, or by not sitting.
My technique is keeping the back locked out and generating power from the legs. I've even experimented with only moving the legs and keeping arms locked out and unused - even that put pressure on my back. I can only presume I have a sensitive back if other people don't have this problem.
So, the only cardio machine I can't use. But I need cardio and I can feel it.
What else can I do? We have 3 flights of 10 step stairs but the stopping and starting with walking doesn't seem to give much effect. I can't run up them as this is a working environment.
Any ideas? I want something I can keep going for just 15mins a day.